The present invention relates to paper machines.
In particular, the present invention relates to hydraulic headboxes of paper machines and to devices for damping the pressure variation of stock suspension in a hydraulic headbox.
Thus, the present invention relates, in a paper machine, to a hydraulic headbox which has an inlet distributor by means of which the pulp suspension flow is distributed as uniformly as possible to a structure which conducts the pulp suspension to the slice of the headbox. This hydraulic headbox cooperates with a gas enclosure such as an air tank the purpose of which is to damp pressure variations which may occur from time to time in the pulp stock.
In general, known headboxes may be divided into three main catagories:
A. headboxes provided with air cushion constructed in immediate connection with the headbox, so as to provide so-called air-cushion headboxes, PA1 B. hydraulic headboxes provided with an air cushion separate from the headbox itself and in which air tanks are located either in the feed-pipe system which feeds the pulp stock in advance of a distribution header, or the air tanks may be situated subsequent to the header, and PA1 C. hydraulic headboxes which have no air cushion at all.
The use of an air cushion in connection with a headbox serves to attempt to equalize pressure variations which occur from time to time in the pulp stock flow prior to the outflow aperture or slice of the headbox. These variations may originate either in the pulp system prior to the headbox or they may be produced in the headbox itself. If such pressure variations are permitted to extend all the way to slice defined between the lips of the headbox, then the pressure variations will cause corresponding variations in the velocity of discharge of the stock jet from the headbox with the result that corresponding variations will occur in the base weight in the pulp web that is formed on the forming wire. Such variations in base weight, occurring longitudinally of the direction of travel of the web in the paper machine, cannot be equalized, at least in their entirety, during subsequent drying of the web, so that these variations will be apparent in the completed paper, thus detracting from the value thereof.
In order to obtain a uniform average flow velocity profile not only in the direction of web travel but also in the cross-machine direction, transversely of the direction of web travel, the inlet pipe of the headbox, or the so-called distribution header thereof, is very often tapered, having the configuration of a truncated cone, and providing for the pulp suspension a cross-section of flow which gradually diminishes from the inlet toward the outlet of the distributor header which is often provided at its outlet with a continuous bypass flow. From this tapered header which extends across the machine there depart at uniform intervals a relatively large number of so-called diffusor pipes which extend longitudinally of the machine and along which the stock flows from the header into the headbox. A geometry of the header which will achieve equal flow velocities and mass flow rates in each and every diffusor pipe can be mathematically calculated only as an approximation. Various defects in form are necessarily introduced into the geometry of the header during the manufacture of the headbox, and in addition because of pressure variations which occur during operation, these errors give rise to different flow velocities in different diffusor pipes, so that as a result there are unavoidable errors preventing achievement of a desired cross-machine velocity profile for the stock suspension flow.
In the first of the above-mentioned types of headboxes, namely the air-cushion headbox referred to under a) above, pressure variations which occur from time to time are usually damped in a highly efficient manner because in such headboxes the stock surface against which the air cushion acts has comparatively large area and the height of the stock space, measured at right angles to the direction of flow, is relatively small. A further advantage of such headboxes is that in general the air cushion extends quite close to the discharge slice or lip of the headbox, so that the opportunity for new pressure fluctuations to be generated in the region between the sphere of action of the air cushion and the discharge slice is minimal.
However, in spite of the above advantages of the air-cushion headbox, in recent times in the newest fast-operating paper machines, these air-cushion headboxes have quite often been replaced by hydraulic or fully hydraulic headboxes of the types (b) and (c) referred to above. The reason why these latter types of hydraulic headboxes have been used to a greater extent in recent times is that they have a greater ease of assembly particularly in connection with the relatively new twin-wire web-forming structures or so-called formers, and on the other hand these hydraulic types of headboxes have lower manufacturing costs. The higher turbulence of the stock jet discharging from the slice and its more favorable intensity distribution, as well as the resulting higher homogeneity of the stock, have also favored the introduction of these hydraulic headboxes.
However, the above advantages of hydraulic headboxes are opposed by certain difficulties encountered thereby, these difficulties being caused primarily by pressure variations in the stock as referred to above. As a result it has often been necessary after a hydraulic headbox has been provided to add to the headbox which originally was intended to operate as a fully hydraulic headbox, one or more separate air tanks which are intended to provide a substitute for the air cushion achieved in the conventional air-cushion type of headbox. Various structural designs are known with respect to the placement of such separate air tanks in connection with hydraulic headboxes. In some of these designs the air tanks are connected to the pulp stock pipe system in advance of the headbox, while in other constructions the air tanks are placed above the headbox itself and connected thereto by suitable connecting pipes or by way of a connecting duct communicating with the upper part of the headbox space. The above first type of construction according to which air tanks are connected to the hydraulic headbox system has the drawback that pressure variations produced before such a tank may perhaps be sufficiently damped but new fluctuations may arise in the region between the air tank and the lip slice of the headbox from various sources of disturbance, such as improper configuration of the distribution header, and these pressure variations now have access, without damping, to the lips at the slice of the headbox where the pressure variations will result in variations in the base weight of the paper as referred to above.
The second type of connection of the air tank to the hydraulic headbox also has an undesirable feature in that when the air tank is placed over the headbox the height of the free liquid level from the central axis of flow thereof is of necessity extremely great, or the connecting pipes or duct between the headbox and the air tank must be dimensioned so as to be relatively narrow as compared with the main flow passage. In both of these situations there is an impaired damping of the pressure fluctuations in the stock suspension as compared with the pressure variation damping capacity capable of being achieved by a conventional air-cushion type of headbox.